1. Field of the Invention
This invention relates to bearings and, more particularly, to bearings having localized compressive residual stresses imparted by laser shock peening along high stress areas of the bearings.
2. Description of Related Art
Conventional bearing assemblies includes inner and outer races between which are positioned a plurality of circumferentially spaced bearing or rolling elements such as ball or roller bearings, which are circumferentially spaced apart by an annular cage. Ball type bearings are commonly found in gas turbine engines where they are used as thrust bearings which typically have the outer race fixedly joined to a stationary housing, and the inner race fixedly mounted to a rotor shaft which is supported thereby. Roller bearings are elongated and are typically used to support concentric rotating shafts of the engine. Roller bearings may be of the cylindrical type or tapered conical type. The bearing elements and the races are subject to stresses.
For high rotational speed applications of the bearing assembly, for example, at DN values on the order of about 1.5.times.10.sup.6, the inner race of the thrust ball bearings is axially split for providing a radially extending center plane disposed generally in line with the bearing centers, with a plurality of circumferentially spaced and radially extending oil center feed passages disposed at the split line. The center feed passages may be used to effectively carry lubricating oil radially outwardly to the bearings under centrifugal force upon rotation of the inner race and shaft. In a high speed split inner race bearing assembly, the inner races are subjected to relatively high stress induced by rotation. In particular, the central zone over which the ball tracks is most sensitive to any additional stress risers caused by material discontinuities such as holes or slots used to feed oil to the roller elements and the race surfaces upon which they bear and which can decrease the effective life of the bearing assembly. Conventional split inner race bearing assemblies, particularly, those used in gas turbine engines may also include secondary radial passages for lubricating adjacent components of the bearing assembly spaced axially from the split line.
U.S. Pat. No. 5,147,140, entitled "Ball-and Roller Bearing", by Murakami et al., provides for shot peened surfaces to impart compressive residual stresses on the order of a minimum of 100 kgf/mm.sup.2 to a depth of up to 2% of the mean diameter of the rolling element. Recognizing the limitations of shot peening and the texturing it forms on the bearing or contact surfaces, the Murakami patent requires a closely sealed bearing using packing grease or some other technique to closely seal the bearing. This is not practical for many applications such as with main rotating shafts in gas turbine engines which require a constant supply of oil for cooling as well as lubrication purposes. It is therefore highly desirable to provide bearings with rolling and static elements having rolling contact surfaces with regions of deep compressive residual stresses beneath and a minimal amount of surface texturing so that sealed bearings are not required such as in the Murakami patent.
The region of deep compressive residual stresses imparted by laser shock peening of the present invention is not to be confused with a surface layer zone of a work piece that contains locally bounded compressive residual stresses that are induced by a hardening operation using a laser beam to locally heat and, thereby, harden the work piece such as that which is disclosed in U.S. Pat. No. 5,235,838, entitled "Method and Apparatus for Truing or Straightening Out of True Work Pieces". The present invention uses multiple radiation pulses from high power pulsed lasers to produce shock waves on surface of transition area of forging dies using methods similar to those disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser peening as understood in the art and as used herein means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". However, the prior art does not disclose laser shock peening of curved contact surfaces of bearings nor the methods of how to produce them. It is to this end that the present invention is directed.